Shielded cables in which at least one central conductor is surrounded by one or more conductive shields are typically employed in environments where it is important to isolate electric circuits from the effects of outside disturbances. These disturbances generally consist of varying electromagnetic fields external to the cables. The disturbances causes currents to flow in the shields. The shield currents, in turn, give rise to voltage disturbances carried over the internal conductor. The shielding is effective to the extent that currents induced in the shield are ineffective in generating disturbances in the inner conductors.
Surface transfer impedance is one measure of shielding effectiveness. As defined, for example, in Oakley, R. J., "Surface Transfer Impedance Measurements--A Practical Aid to Communication Cable Shielding Design," Proceedings of the 18th International Wire and Cable Symposium, Atlantic City, December 1969 (Lachine, Quebec, Canada, Northern Electric Co., 1969), surface transfer impedance is the magnitude of the ratio per unit length of the longitudinal induced voltage along a length on the inside surface of the shield to the current made to flow in a circuit including the shield and a return path outside of the shield. The voltage on the inside of the shield is, of course, induced by the current flowing on the outside of the shield.
In the prior art, it was considered easier to measure shielding effectiveness without obtaining the absolute value of the surface transfer impedance as a function of frequency, as exemplified by U.S. Pat. No. 3,839,672 issued to Anderson and assigned to the assignee of the present invention. The Anderson invention has been implemented in a testing system that measures the signal level of a signal radiated by a piece of cable relative to the signal level of a reference signal applied to the cable.
Recent federal regulations, in particular, 47 CFR Part XV, require measurements of the absolute values of systems radiated emissions rather than the relative shielding effectiveness as obtained by the Anderson method. Cable radiated emissions may be calculated from measured values of surface transfer impedance. There is accordingly need for systems useful for making rapid and economical surface transfer impedance measurements.
Surface transfer impedance has been determined in accordance with the theoretical definition by measuring the voltage generated along a length of cable between the shield and an inner conductor of the cable resulting from a current flowing in the shield with a return path outside of the shield. Practical measurements were made in the past by placing a length of cable inside of a conducting tube. The conducting tube was connected to the cable shield to provide the requisite external circuit through which current could be passed. The resulting induced voltage was then measured at one end of the sample.
It is necessary in making such measurements to limit carefully the section of sample in which current is permitted to flow in order to define the length over which transfer impedance is measured. It is also necessary to control the current flow direction in order to determine the input current which is effective in generating the measured output voltage so that the measured impedance is substantially in accord with its theoretical definition.
In the prior art as described by Simons, K. A., "A Review of Measuring Techniques for Determining the Shielding Efficiency of Coaxial Cables," IEC Doc. SC46A/WG1(1973) p. 15, the current conduction distance was limited at low frequencies by short-circuiting one end of the shield to the conducting tube and applying the input current to the other end. This method was satisfactory in a frequency regime ranging up to about 30 MHz.
Above about 30 MHz, the method just described gave rise to difficulties because the conducting tube and sample shield acting together produced standing waves which distorted the measured test results. Therefore, prior art high frequency transfer impedance measurements were made by blocking the ends of the cavity between the tube and shield with parallel resistances and toroids having high series loss as described by Simons pp. 15-20. This technique is called the terminated triaxial method. The toroids and a parallel resistor effectively terminated the transmission line formed by the tube and the cable shield into its characteristic impedance. The cable shield is connected to the conducting tube at points outside of the section blocked by the toroids. The connections of the shield to the conducting tube outside of the toroids, as required by the terminated triaxial method, required that the low frequency limit of the high frequency test range be not less than 3-10 MHz. The terminated triaxial method is known to be useable up to frequencies of about 1 GHz.
Therefore, one of the problems associated with surface transfer impedance measurements in the prior art has been that different test sets have been required for different frequency regimes. It is convenient, however, to have two completely different test configurations for low frequency (up to 30 MHz) and high frequency (say above 1 to 5 MHz) measurements.
A second problem in the terminated triaxial method is associated with the necessity of pulling the sample through the conducting tubes and toroids. As a result, generally, the connectors can be attached to the sample only after inserting the sample into the fixture. Hence, the sample with connectors cannot be prepared beforehand, thereby making it difficult to conduct mass tests in industrial applications or to test assemblies with permanently mounted connectors. The need for repetitive pulling and connector soldering makes it difficult for the operator to perform the necessary changes, adjustments, and mechanical manipulations without affecting the test conditions of the samples. These are especially important in performance stability measurements and research applications. Finally, the terminated triaxial method involves a time-consuming procedure of installing and removing the samples in the fixture and connecting cable shielding to probes in the chamber.